The dynamic properties of cells membranes is the principal focus of this project. The most recent studies have examined spontaneous mechanisms of lipid bilayer assembly in a system that consists of synthetic and naturally occurring phospholipids dispersed in water. The lipid bilayer is generally accepted as the principle lipid structure in cell membranes. Equilibrium techniques have been developed that measure the chemical potentials of the lipids as a function of temperature. In principle, these phase diagrams provide a general framework for describing all lipid states that form spontaneously. This approach has led to the finding of the spontaneous formation of a single lipid bilayer state that exists at a singularity in temperature. This state has been shown to exist at a critical temperature (BCT). The BCT has been described with respect to the chemical structure of the phospholipid (polar and hydrocarbon chain moieties) and to the dependence of the BCT on mixtures of phospholipids. As a test of the relevance of the BCT to the assembly of lipid bilayers in cell membranes, bacteria were grown at different temperatures; the membrane lipids were extracted, and dispersed in water; and the BCT for each of these lipid mixtures was obtained. For all cell systems that were studied the BCT was identical with the temperature of cell growth within an experimental error of 1.0C. Thus B. stearothermophilus grown at 49C and 59C, and E. coli grown at 20C and 30C exhibited BCT's at these same temperatures. Since the BCT is defined only by the temperature and composition of the membrane lipid extract, this constitutes a thermodynamic proof that the phase relations of the lipids in the cell membranes are those of the bilayer critical state. In principle, therefore, these cell membranes utilize the same process for the assembly of lipid bilayers.